EXPLORING THE ATOM
Events > Atomic Discoveries, 1890s-1939
In 1919, New Zealander Ernest Rutherford reported on a series of experiments he had been conducting in Manchester.
Rutherford found that nitrogen nuclei ejected what he suspected was "a hydrogen atom" when bombarded with energetic α (alpha) particles.
Subsequently, he named this fundamental particle the proton.
The final addition to the atomic "miniature solar system", first proposed by Niels Bohr, came in 1932 when James Chadwick, Rutherford's colleague at Cambridge,
identified the third and final basic particle of the atom: the neutron.
By the early 1930s, the atom was thought
to consist of a positively charged nucleus, containing both protons
and neutrons, circled by negatively charged electrons equal in number to the protons in the nucleus.
The number of protons determined the element's atomic number. Hydrogen, with one proton, came
first and uranium, with ninety-two protons, last
on the periodic table. This simple scheme became more complicated when chemists discovered that
many elements existed at different weights even while displaying identical chemical properties. It was Chadwick's
discovery of the neutron in 1932 that explained this mystery. Scientists found that the weight
discrepancy between atoms of the same element resulted because they contained different numbers
of neutrons. These different classes of atoms
of the same element but with varying numbers of neutrons were designated isotopes. The three
isotopes of uranium found in nature, for instance, all have ninety-two protons in their nuclei and ninety-two electrons
in orbit. But uranium-238, which accounts for over ninety-nine percent of natural uranium, has 146
neutrons in its nucleus, compared with 143 neutrons in the rare uranium-235 (.7 percent of natural
uranium) and 142 neutrons in uranium-234, which is found only in traces in the heavy metal. The
slight difference in atomic weight between the uranium-235 and uranium-238 isotopes figured greatly
in nuclear physics during the 1930s and 1940s.
The year 1932 produced other notable
events in atomic physics. The Englishman J. D. Cockcroft and the Irishman E. T. S. Walton, working jointly at the
Cavendish Laboratory, were the first to split the atom when they bombarded lithium with protons generated
by a type of particle accelerator
(dubbed a "Cockcroft-Walton machine") and changed the resulting
lithium nucleus into two helium nuclei. Also in that year, Ernest O. Lawrence and
his colleagues M. Stanley Livingston and Milton White successfully
operated the first cyclotron at the University of
California, Berkeley (right).
Sources and notes for this page.
The text for this page was adapted from, and portions were taken directly from, the Office of History and Heritage Resources publication:
F. G. Gosling, The Manhattan Project: Making the Atomic Bomb (DOE/MA-0001; Washington: History Division, Department of Energy, January 1999), 1.
The photograph of Ernest Rutherford (and James Chadwick in the background) is courtesy the Lawrence Berkeley National Laboratory.
The atom graphic is a combination of graphics that were originally produced by the Washington State Department of Health
(the nucleus) and the Environmental Protection Agency (everything else);
the combination of the two graphics, the labels, and other customizations, are original to the Department of Energy's Office of History and Heritage Resources.
The photograph of the cyclotron at the "Rad Lab," and its caption, are courtesy the National Archives.
Click here for more information on the comic book images.
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